Field of the Invention
The present invention relates to a grating objective having a first and a second diffraction grating arranged one behind the other in the radiation path extending from the object side to the image side. The gratings are adapted and arranged in such a way that a beam ray which is an axial ray or a border ray for one grating is a border ray or an axial ray, respectively, for the other grating. The invention also relates to a beam shaper for reshaping the cross-section of a radiation beam. The invention further relates to an optical scanning device having the grating objective and/or the beam shaper.
A grating objective of the type described above is known from the article "Wavelength independent grating lens system" in "Applied Optics", Vol. 28, no. 4, 1989, pages 682-686. As compared with conventional objective lenses, objectives in the form of gratings or holograms have the advantage that they are smaller and lighter and can be more easily mass-produced by means of known replica techniques. The drawback of a grating used as a lens is that it has a greater wavelength dependence than a conventional lens. At a small variation of the wavelength of the beam, the angle at which the beam portions are diffracted changes, which results in image aberrations.
To reduce the wavelength dependence, this article in "Applied Optics", 1989, pages 682-6 proposes to compose a grating lens from two gratings which are arranged one behind the other. The gratings are adapted in such a way that each ray of a beam is diffracted by the second grating at a second angle which is opposed to a first angle at which this ray is diffracted by the first grating. Consequently, the deviation of the first diffraction angle, caused by a wavelength variation, is compensated by an opposite deviation of the second diffraction angle.
The grating lens described in the article in "Applied Optics", 1989, pages 682-6 is sufficiently achromatic, or wavelengthinvariant, but this grating has a very small image field, for example, with a diameter of the order of 1 .mu.m. Consequently, this lens is not suitable for certain applications. One of these applications is its use as an objective in a device for scanning an information plane in an optical record carrier. This objective must focus a radiation beam from a radiation source, for example a diode laser such as an AlGaAs laser, to a diffraction-limited radiation spot on an information track in the information plane. This objective must have a relatively large diffraction-limited image field, with a diameter of the order of 100 .mu.m, or a field angle of the order of 1.degree.. The larger image field is necessary so that the scanning device can be adjusted during its assembly, i.e. to enable the various components to be positioned satisfactorily with respect to each other and to enable the scanning beam to be tilted with respect to the objective during use of the device so that the scanning spot position can be corrected with respect to a track pattern to be scanned in the information plane. As already noted in the article in "Applied Optics", 1989, pages 682-6, the objective described in this article produces a beam having a non-uniform, i.e. a Gaussian intensity distribution.